Polyimide film, image-forming apparatus, method for producing the polyimide film, and method for producing the intermediate transfer belt

ABSTRACT

A polyimide film having a polyimide as the primary constituent and containing at least a polymeric electroconductive material, wherein in a weight-decrease vs. temperature-increase curve when the polyimide film is heated gradually from 30° C., a weight-decrease rate at a first inflection point, where an amount of difference in weight-decrease rates decreases first from 30° C., is in the range of −1.9% or more and −1.4% or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a polyimide film having a polyimide as theprimary constituent and containing at least a polymericelectroconductive material, an image-forming apparatus having anintermediate transfer belt made of the polyimide film, a method forproducing the polyimide film, and a method for producing theintermediate transfer belt.

2. Description of the Related Art

Some image-forming apparatuses by electrophotographic process employ anendless intermediate transfer belt (see, e.g., Japanese PatentApplication Laid-Open (JP-A) No. 62-206567 ). In the image-formingapparatuses equipped with an intermediate transfer belt, an image isformed by forming an electrostatic latent image on the surface of animage carrier, developing the electrostatic latent image with a tonerinto a toner image, transferring the toner image on the image carrieronce onto the intermediate transfer belt electrostatically (primarytransfer) by applying a bias voltage to the intermediate transfer beltcirculating while in contact with the image carrier, finallyretransferring the toner image on the intermediate transfer belt onto arecording paper and fixing the image thereon into a fixed toner image(secondary transfer),

The intermediate transfer belt is an endless element prepared by cuttinga film-shaped base material into a long belt and connecting both ends ofthe belt to each other. Materials for use as the intermediate transferbelts hitherto proposed include conductive belts made of thermoplasticresin including a polycarbonate resin (see JP-A No. 6-95521);polyvinylidene fluoride (PVDF) (see JP-A Nos. 5-200904 and 6-228335);polyalkylene phthalate (see JP-A No. 6-149081), and blends ofpolycarbonate (PC) /polyalkylene terephthalate (PAT) (see JP-A No.6-149083), ethylene tetrafluoroethylene copolymer (ETFE)/PC, ETFE/PAT,and PC/PAT (see JP-A No. 6-149079); and the like.

However, a conductive material made of a thermoplastic resin such as apolycarbonate resin, polyvinylidene fluoride (PVDF) or the like isinferior in mechanical properties, leading to larger deformation of thebelt by the stress during operation and consequently to less reliableformation of high-quality transferred images. In addition, it also has ashortened belt life, due to cracks generated at the belt edges duringoperation.

Alternatively, intermediate transfer belts made of a thermosettingpolyimide resin, which is superior in mechanical properties and heatresistance, and conductive filler dispersed therein is proposed (seeJP-A Nos. 5-77252 and 10-63115). An intermediate transfer belt made of acarbon black-dispersed thermosetting polyimide was also proposed (seeJP-A No. 63-311263).

However, such intermediate transfer belts carry a problem that theintermediate transfer belts made of a thermosetting polyimide resincontaining a conductive filler or carbon black as the conductivematerial exhibit decrease in volume or surface resistivity by repeatedapplication of bias voltages during the primary transfer, and thus couldnot provide high-quality transfer images reliably. They also carry adisadvantage that if a conductive filler or carbon black is dispersed inthe polyimide resin, the resulting belt becomes brittle and morevulnerable to cracks.

As a new measure to overcome these disadvantages, an intermediatetransfer belt having a polyimide as the primary constituent andcontaining at least polyaniline and a dopant for making the polyanilineconductive was proposed (see JP-A No. 2001-109277).

SUMMARY OF THE INVENTION

However, although the intermediate transfer belt described in PatentReference 11 containing a dopant for making the polyaniline conductivehas extremely consistent volume and surface resistivity during repeatedapplication of bias voltages compared to the intermediate transfer beltsdescribed in Patent References 8 to 10 containing a conductive filler orcarbon black dispersed therein if the polyaniline is made sufficientlyconductive until it reaches a stabilized region, but if the conductivityof the belt is insufficient, the volume resistivity and surfaceresistivity gradually deteriorate over time regardless of whether thebias voltages are applied repeatedly or not. Another disadvantage isthat it requires an extended period of time to make polyanilinesufficiently conductive. Yet another disadvantage is that the beltbecomes shrunk and deformed by the heat applied when the cut belt isthermally compressed at both ends in production of the intermediatetransfer belt, resulting in deterioration in dimensional accuracy andthus could not provide high-quality transfer images reliably.

The present invention has been made in view of the above circumstancesand provides a polyimide film containing a polymeric electroconductivematerial sufficiently higher in conductivity and thus having astabilized resistivity, an image-forming apparatus having anintermediate transfer belt made of the polyimide film, a method forproducing the polyimide film in a period as short as possible, and amethod for producing the intermediate transfer belt that provideshigh-quality transfer images.

The polyimide film according to the invention is a polyimide film havinga polymeric electroconductive material as the primary constituent andcontaining at least a polymeric electroconductive material, wherein

in a weight-decrease vs. temperature-increase curve when the polyimidefilm is heated gradually from 30° C., a weight-decrease rate at anintersection between a tangent line which passes a first maximum pointin the curve having the maximum value in the slope and a tangent linewhich passes a minimum point in the curve having the minimum value inthe slope after the first point, is in the range of −1.9% or more and−1.4% or less.

Final polyimide films are normally produced after baked in an oven orthe like. The polyimide films immediately after discharge from an ovendo not contain water and absorb water over time as the polyimide filmsare inherently hygroscopic. On the contrary, polyimide films containingwater also release water when placed in a low-temperature andlow-humidity environment. Polyimide films stored in normal environment(e.g. temperature: 22° C., and humidity: 55% RH) for a long period havestabilized water content, but release water and lose weight when heatedgradually.

In particular, in the case of polyimide films containing polyaniline asthe polymeric electroconductive material, the polyaniline therein ismade more conductive by oxidation. Although polyaniline, a hygroscopiccompound, demands presence of some amount of water for oxidation, ifwater is present in an excess amount, it becomes lower in resistivitydue to the ionic conduction caused by the residual ion components fromthe polyaniline and dopant.

Considering the normal environment where the polyimide films are used(temperature: approximately 22° C., and humidity: around 55% RH), theallowable range of the weight-decrease rate thereof during a period from30° C. to the first inflection point in the weight decrease vs.temperature curve when the polyimide film is heated gradually from 30°C. is specified here in the invention. If the weight-decrease rate isless than −1.9% or more than −1.4%, the water content of polyimide filmsvaries and the resistivity of polyimide films becomes unstable in normalenvironment where polyimide films are used. In other words, the watercontent of the polyimide film according to the invention andconsequently the resistivity thereof become more stabilized in normalenvironment.

The image-forming apparatus according to the invention is animage-forming apparatus that forms an image by forming a toner image onan image carrier and transferring and fixing the toner image finally ona recording medium, including an intermediate transfer belt in contactwith the image carrier whereon the toner image is formed for receivingthe toner image from the image carrier once by application of a biasvoltage and transferring the toner image onto a recording medium,wherein, the intermediate transfer belt is made of a polyimide filmhaving a polyimide as the primary constituent and containing at least apolymeric electroconductive material, and in a weight-decrease vs.temperature-increase curve when the the intermediate transfer belt isheated gradually from 30° C., a weight-decrease rate at an intersectionbetween a tangent line which passes a first maximum point in the curvehaving the maximum value in the slope and a tangent line which passes aminimum point in the curve having the minimum value in the slope afterthe first point, is in the range of −1.9% or more and −1.4% or less.

In the image-forming apparatus according to the invention, the surfaceresistivity and volume resistivity of the intermediate transfer belt arestable even after repeated application of bias voltages and thus allowreliable production of high-quality transfer images.

The intermediate transfer belt may have a surface resistivity of 10¹⁰Ω/□ or more and 10¹⁴ ≠/□ or less and a volume resistivity of 10⁸ Ω·cm ormore and 10¹⁴ Ω·cm or less, and the intermediate transfer belt maycontain filler particles having a particle diameter of 0.1 μm or moreadditionally added.

The former polyimide film provides high-quality transfer images, whilethe latter suppresses fluctuation in the length of intermediate transferbelt and provides the intermediate transfer belt with sufficiently highmechanical strength, assuring more stabilized movement of the belt. Eachresistivity above is the value as determined when a voltage of 100V isapplied.

The method for producing a polyimide film according to the inventionincludes the steps of forming a polyimide compound having a polyimide asthe primary constituent and containing at least a polymeric conductiveagent into a film shape; and exposing to high temperature and humiditythe film-shaped polyimide compound formed in the step of forming thepolyimide compound wherein the polyimide compound is kept in anenvironment of a temperature of 27° C. or more and a humidity of 70% RHor more.

By the method for producing a polyimide film according to the invention,the water content of the polyimide film is stabilized in a shorterperiod of time by the processing in the step of high temperature andhumidity exposure, allowing more efficient production of the polyimidefilm according to the invention.

The method for producing an intermediate transfer belt according to theinvention is a method for producing an endless intermediate transferbelt in contact with an image carrier whereon a toner image is formedfor receiving the toner image once from the image carrier by applicationof a bias voltage and retransferring the toner image onto a recordingmedium, for use in an image-forming apparatus wherein an image is formedby forming a toner image on an image carrier and retransferring andfixing the toner image finally on a recording medium, including thesteps of:

exposing a film-shaped polyimide compound to high temperature andhumidity at a temperature of 27° C. or more and a humidity of 70% RH ormore, the film-shaped polyimide compound having a polyimide as theprimary constituent, containing at least a polymeric electroconductivematerial and having a differential thermal characteristic in which adifferential thermal curve is created, the differential thermal curve ata temperature of 150° C. or more having a plurality of inflection pointscorresponding to an intersection between a tangent line passing a pointof a change from increase to decrease in the slope of the curve and atangent line passing a point of a change from decrease to increase inthe slope of the curve;

cutting into a long belt the film-shaped polyimide compound after thestep of exposure to high temperature and humidity;

and heat-compressing the resultant belt by connecting both ends of thebelt cut off in the step of cutting into a long belt with an adhesiveand heating and compressing both connected end regions at a temperaturein the range of 150° C. or more to the inflection point closest to 150°C. where the slope of the differential thermal curve becomes smaller.

By the method for producing an intermediate transfer belt according tothe invention, the resistivity of the intermediate transfer belt becomesstabilized in a shorter period of time by the processing in the step ofhigh temperature and humidity exposure, and additionally an intermediatetransfer belt superior in dimensional accuracy can be obtained by theprocessing in the step of thermocompression. That is, if a wet belt issubjected to thermocompression, water in the belt is released, causingshrinkage of the belt in the width direction and waveness on the surfacethereof. If the temperature of thermocompression is in the range of 150°C. or more to the inflection point closest to 150° C. where the slope ofthe differential thermo curve becomes smaller, the heat-compressed beltwhich is suppressed in the shrinkage in the width direction and surfacewaveness, returns almost to the original shape by absorbing water in theenvironment when allowed to cool, allowing the shrunk portion to returnto the original shape, and thus can regain its favorable dimensionalaccuracy, but when the temperature of thermocompression is higher thanthe temperature above, the belt becomes completely or almost absolutelydry, leaving the shrunk area and the worsened dimensional accuracy asthey are.

The invention provides a polyimide film overcoming the disadvantagesdescribed above, i.e., a polyimide film having a polymericelectroconductive material sufficiently higher in conductivity and thusmore stabilized in resistivity, an image-forming apparatus having anintermediate transfer belt made of the polyimide film, a method forproducing the polyimide film in a period as short as possible, and amethod for producing the intermediate transfer belt that provideshigh-quality transfer images.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail based on the following figures wherein

FIG. 1 is a view illustrating an embodiment of a polyimide filmaccording to the invention, a semiconductive polyimide film;

FIG. 2 is a graph showing results of thermogravimetric analysis of thesemiconductive polyimide film shown in FIG. 1;

FIG. 3 is a flow chart illustrating a process for producing thesemiconductive polyimide film shown in FIG. 1;

FIG. 4 is a schematic view illustrating an embodiment of animage-forming apparatus according to the invention;

FIG. 5 is a flow chart illustrating a method of producing theintermediate transfer belt shown in FIG. 4;

FIG. 6 is a view illustrating the end regions of a belt cut off in thelongitudinal direction in cutoff step;

FIG. 7 is a view illustrating a way of connecting both terminals of thebelt shown in FIG. 6;

FIG. 8 is a graph showing results of differential scanning calorimetryof the belt shown in FIG. 6;

FIG. 9 is a perspective view illustrating an intermediate transfer beltproduced via respective steps shown in FIG. 5;

FIG. 10 is a schematic view illustrating the configuration of animage-forming apparatus equipped with a rotary developing device;

FIG. 11 is a graph showing the results of measuring the surfaceresistivity and volume resistivity in EXAMPLE 1-1;

FIG. 12 is a graph showing the results of measuring the change over timein the surface resistivity and volume resistivity of a sampleconditioned at 28° C. and 85% RH for one week;

FIG. 13 is a graph showing the results of measuring the surfaceresistivity and volume resistivity in EXAMPLE 2;

FIG. 14 is a graph showing results of measuring the change over time inthe surface resistivity and volume resistivity of a sample conditionedat 48° C. and 85% RH for 18 hours;

FIG. 15 is a graph showing the results of measuring the surfaceresistivity and volume resistivity in COMPARATIVE EXAMPLE 1; and

FIG. 16 is a graph showing the results of measuring the surfaceresistivity and volume resistivity in EXAMPLE 3.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention will be described withreference to drawings.

FIG. 1 is a view illustrating a semiconductive polyimide film, anembodiment of a polyimide film according to the invention.

The semiconductive polyimide film shown in FIG. 1 has polyimide as theprimary constituent and contains polyaniline as a polymericelectroconductive material and a dopant for making the polyanilineconductive.

Results of the thermogravimetric analysis of the semiconductivepolyimide film shown in FIG. 1 will be described below.

FIG. 2 is a graph showing the results of thermogravimetric analysis ofthe semiconductive polyimide film shown in FIG. 1.

A simultaneous differential thermal and thermogravimetric analyzerDTG-50 manufactured by Shimadzu Corporation was used for thethermogravimetric analysis. In measurement, the sample was heated fromroom temperature at a heating rate of 10° C./min, kept at 30° C. for 20minutes, and then heated up to 300° C. The analysis was performed in airwithout use of nitrogen gas. The temperature and the weight-decreaserate data were obtained at an interval of 1 second.

The temperatures and the weight-decrease rates (%) are plottedrespectively on an abscissa axis and on an ordinate axis in the graphshown in FIG. 2. In the graph of FIG. 2, shown is a thermalweight-decreasing curve wherein an inflection point wp corresponding tothe intersection between a tangent line (c) at the first point (a) inthe curve having the maximum value in the slope of curve and a tangentline (d) at the second point (b) in the curve having the minimum valuein the slope of curve is indicated. The inflection point wp is the firstinflection point where the amount of change in the weight-decrease ratesdecreases when determined from a temperature of 30° C., and thetemperature of the inflection point wp is approximately 140° C. Theweight-decrease rate at a temperature (approximately 140° C.) of thisinflection point wp is approximately −1.65%. If a semiconductivepolyimide film has a weight-decrease rate at the first inflection point,where the amount of change in the weight-decrease rates decreases from30° C., in the range of −1.9% or more and −1.4% or less, the watercontent of the film is stable in normal environment where this film isused (temperature: approximately 22° C.; and humidity: approximately 55%RH), and accordingly the resistivity of polyimide film is stabilized.

Next, the process for producing the semiconductive polyimide film shownin FIG. 1 will be described.

FIG. 3 is a flow chart illustrating the process for producing thesemiconductive polyimide film shown in FIG. 1.

For production of the semiconductive polyimide film shown in FIG. 1, afilm-shaped polyimide compound is first prepared in film-forming step(Step S11). In the film-forming step, the film-shaped polyimide compoundmay be produced, for example, by forming a long film by extruding acoating solution containing a polyamic acid, polyaniline, dopant andsolvent via a T die, for example, onto a favorably exfoliated basematerial which is also favorable as a model (e.g., stainless-steelendless belt) and then imidizing the resulting film according to a knownmethod into a film-shaped polyimide compound, or by coating the coatingsolution onto the internal surface of a cylindrical metal mold accordingto a known method while rotating the metal mold for leveling the filmthickness and drying partially and then completely.

The coating solution contains polyamic acid as the primary raw material,polyaniline as the polymeric electroconductive material, and a dopantfor making the polyaniline electrically conductive.

As described in JP-A No. 2001-109277, the polyamic acid can be obtainedas a solution, by dissolving a near-equimolar mixture of atetracarboxylic dianhydride or a derivative thereof and a diamine in apolar organic solvent and allowing these compounds to react in thesolution. The tetracarboxylic dianhydride may be an aromatictetracarboxylic dianhydride, while the diamine is an aromatic diamine,but the other compounds may be selected suitably as needed.

Examples of the aromatic tetracarboxylic dianhydrides includepyromellitic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylicdianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride,2,3,3′,4′-biphenyltetracarboxylic dianhydride,2,3,6,7-naphthalenetetracarboxylic dianhydride,1,2,5,6-naphthalenetetracarboxylic dianhydride,1,4,5,8-naphthalenetetracarboxylic dianhydride,2,2′-bis(3,4-dicarboxyphenyl)propane dianhydride,bis(3,4-dicarboxyphenyl)sulfone dianhydride, and the like. Thesedianhydrides may be used alone or in combination of two or more.

Examples of the aromatic diamines include 4,4-diaminodiphenylether,4,4′-diaminodiphenylmethane, 3,3′-diaminodiphenylmethane,p-phenylenediamine, m-phenylenediamine, benzidine,3,3′-dimethoxybenzidine, 4,4′-diaminodiphenylsulfone,4,4-diaminodiphenylsulfide, 4,4′-diaminodiphenylpropane,2,2-bis[4-(4-aminophenoxy)phenyl]propane, and the like. These diaminestoo may be used alone on in combination of two or more.

It is acceptable to use a polyaniline having the quinonediiminestructural unit or the phenylenediamine structural unit represented bythe following General Formula (1) as the primary recurring unit, where mand n represent respectively the molar fractions of the quinonediimineand phenylenediamine structural units in its recurring unit, and 0<m<1;0<n<1; and m+n=1.[Formula 1]

Normally, it is acceptable to use a protonic acid as the dopant.Preferable protonic acids favorable for used as the dopant have an aciddissociation constant pKa of 4.8 or less. Examples of such protonicacids include inorganic acids such as hydrochloric acid, sulfuric acid,nitric acid, phosphoric acid, borofluoric acid, phosphofluoric acid, andperchloric acid; and organic acids having an acid dissociation constantpKa of 4.8 or less.

The organic acid is, for example, an organic carboxylic acid or phenolacceptably having an acid dissociation constant pKa of 4.8 or less.Examples of the organic acids include monovalent or polyvalentaliphatic, aromatic, aromatic aliphatic, alicyclic, and the other acids.These organic acids may have a hydroxyl, halogen, nitro, cyano, amino,or other group. Typical examples of these organic acids include aceticacid, n-butyric acid, pentadecafluorooctanoic acid, pentafluoroaceticacid, trifluoroacetic acid, trichloroacetic acid, dichloroacetic acid,monofluoroacetic acid, monobromoacetic acid, monochloroacetic acid,cyanoacetic acid, acetylacetic acid, nitroacetic acid, triphenylaceticacid, formic acid, oxalic acid, benzoic acid, m-bromobenzoic acid,p-chlorobenzoic acid, m-chlorobenzoic acid, p-chlorobenzoic acid,o-nitrobenzoic acid, 2,4-dinitrobenzoic acid, 3,5-dinitrobenzoic acid,picric acid, o-chlorobenzoic acid, p-nitrobenzoic acid, m-nitrobenzoicacid, trimethylbenzoic acid, p-cyanobenzoic acid, m-cyanobenzoic acid,thymol blue, salicyclic acid, 5-aminosalicyclic acid, o-methoxybenzoicacid, 1,6-dinitro-4-chlorophenol, 2,6-dinitrophenol, 2,4-dinitrophenol,p-hydroxybenzoic acid, bromophenol blue, mandelic acid, phthalic acid,isophthalic acid, maleic acid, fumaric acid, malonic acid, tartaricacid, citric acid, lactic acid, succinic acid, α-alanine, β-alanine,glycine, glycol acid, thioglycol acid, ethylenediamine-N,N′-diaceticacid, ethylenediamine-N,N,N′,N′-tetraacetic acid, and the like.

In addition, the organic acid may have a sulfonic or sulfuric acidgroup. Examples of the organic acids include aminonaphtholsulfonic acid,metanilic acid, sulfanilic acid, allylsulfonic acid, laurylsulfuricacid, xylenesulfonic acid, chlorobenzenesulfonic acid, methanesulfonicacid, ethanesulfonic acid, 1-propanesulfonic acid, 1-butanesulfonicacid, 1-hexanesulfonic acid, 1-heptanesulfonic acid, 1-octanesulfonicacid, 1-nonanesulfonic acid, 1-decanesulfonic acid, 1-dodecanesulfonicacid, benzenesulfonic acid, styrenesulfonic acid, p-toluenesulfonicacid, naphthalenesulfonic acid, ethylbenzenesulfonic acid,propylbenzenesulfonic acid, butylbenzenesulfonic acid,pentylbenzenesulfonic acid, hexylbenzenesulfonic acid,heptylbenzenesulfonic acid, octylbenzenesulfonic acid,nonylbenzenesulfonic acid, decylbenzenesulfonic acid,undecylbenzenesulfonic acid, dodecylbenzenesulfonic acid,pentadecylbenzenesulfonic acid, octadecylbenzenesulfonic acid,diethylbenzenesulfonic acid, dipropylbenzenesulfonic acid,dibutylbenzenesulfonic acid, methylnaphthalenesulfonic acid,ethylnaphthalenesulfonic acid, propylnaphthalenesulfonic acid,butylnaphthalenesulfonic acid, pentylnaphthalenesulfonic acid,hexylnaphthalenesulfonic acid, heptylnaphthalenesulfonic acid,octylnaphthalenesulfonic acid, nonylnaphthalenesulfonic acid,decylnaphthalenesulfonic acid, undecylnaphthalenesulfonic acid,dodecylnaphthalenesulfonic acid, pentadecylnaphthalenesulfonic acid,octadecylnaphthalenesulfonic acid, dimethylnaphthalenesulfonic acid,diethylnaphthalenesulfonic acid, dipropylnaphthalenesulfonic acid,dibutylnaphthalenesulfonic acid, dipentylnaphthalenesulfonic acid,dihexylnaphthalenesulfonic acid, diheptylnaphthalenesulfonic acid,dioctylnaphthalenesulfonic acid, dinonylnaphthalenesulfonic acid,trimethylnaphthalenesulfonic acid, triethylnaphthalenesulfonic acid,tripropylnaphthalenesulfonic acid, tributylnaphthalenesulfonic acid,camphorsulfonic acid, acrylamide-t-butylsulfonic acid, and the like.

Alternatively, the organic acid may be a multifunctional organicsulfonic acid having two or more sulfate groups in the molecule.Examples of the multifunctional organic sulfonic acids includeethanedisulfonic acid, propanedisulfonic acid, butanedisulfonic acid,pentanedisulfonic acid, hexanedisulfonic acid, heptanedisulfonic acid,octanedisulfonic acid, nonanedisulfonic acid, decanedisulfonic acid,benzenedisulfonic acid, naphthalenedisulfonic acid, toluenedisulfonicacid, ethylbenzenedisulfonic acid, propylbenzenedisulfonic acid,butylbenzenedisulfonic acid, dimethylbenzenedisulfonic acid,diethylbenzenedisulfonic acid, dipropylbenzenedisulfonic acid,dibutylbenzenedisulfonic acid, methylnaphthalenedisulfonic acid,ethylnaphthalenedisulfonic acid, propylnaphthalenedisulfonic acid,butylnaphthalenedisulfonic acid, pentylnaphthalenedisulfonic acid,hexylnaphthalenedisulfonic acid, heptylnaphthalenedisulfonic acid,octylnaphthalenedisulfonic acid, nonylnaphthalenedisulfonic acid,dimethylnaphthalenedisulfonic acid, diethylnaphthalenedisulfonic acid,dipropylnaphthalenedisulfonic acid, dibutylnaphthalenedisulfonic acid,naphthalenetrisulfonic acid, naphthalenetetrasulfonic acid,anthracenedisulfonic acid, anthraquinonedisulfonic acid,phenanthrenedisulfonic acid, fluorenonedisulfonic acid,carbazoledisulfonic acid, diphenylmethanedisulfonic acid,biphenyldisulfonic acid, terphenyldisulfonic acid, terphenyltrisulfonicacid, naphthalenesulfonic acid-formaline condensate,phenanthrenesulfonic acid-formaline condensate, anthracenesulfonicacid-formaline condensate, fluorenesulfonic acid-formaline condensate,carbazolesulfonic acid-formaline condensate, and the like. These sulfategroups may be present at any positions of the aromatic rings above.

Yet alternatively, the organic acid may be a polymer acid. Examples ofthe polymer acids include polyvinylsulfonic acid, polyvinylsulfuricacid, polystyrenesulfonic acid, sulfonated styrene-butadiene copolymer,polyallylsulfonic acid, polymethallylsulfonic acid,poly-2-acrylamide-2-methyl propanesulfonic acid, poly-haloganatedacrylic acid, polyisoprene sulfonic acid, N-sufoalkylated polyanilines,ring-sufonated polyanilines, and the like. Fluorine-containing polymersknown as Nafion (trademark registered by du Pont, U.S.) may also be usedfavorably as the polymer acid.

In addition, among the esters from the organic acid and the polyhydroxycompound described above, organic esters having an acid terminal arealso acceptable. Examples of these polyhydroxy compounds includepolyvalent alcohols such as ethylene glycol, propylene glycol,1,3-propanediol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol,neopentylglycol, 1,6-hexanediol, 1,4-bis(hydroxyethyl)cyclohexane,bisphenol A, hydrogenated bisphenol A, hydroxypivalyl hydroxypivalate,trimethylol ethane, trimethylol propane,2,2,4-trimethyl-1,3-pentanediol, glycerin, hexanetriol,tris(2-hydroxyethyl)isocyanurate and pentaerythritol; polyether glycolssuch as polyoxyethylene glycol, polyoxypropylene glycol, polyoxyethylenetetramethylene glycol, polyoxypropylene tetramethylene glycol,polyoxyethylene polyoxypropylene polyoxytetramethylene glycol; modifiedpolyether polyols prepared by ring-opening polymerization of thepolyvalent alcohols with ethylene oxide, propylene oxide,tetrahydrofuran, ethyl glycidyl ether, propyl glycidyl ether, butylglycidyl ether, phenyl glycidyl ether or allyl glycidyl ether or thelike; and the like.

Solvents for the acids are, for example, dimethylacetamide (DMAc) andN-methylpyrrolidone (NMP).

The film prepared in the film-forming step, i.e., Step S11 shown in FIG.3, is then subjected to a high temperature and humidity exposure step(Step S12). In the high temperature humidity exposure step, thefilm-shaped polyimide compound prepared in the film-forming step is keptin an environment of a temperature of 45° C. and a humidity of 85% RHfor 20 hours. The polyimide compound absorbs water in the hightemperature and humidity exposure step. The polyimide compound becomesmore oxidized by absorbing water and the polyaniline becomes moreconductive. It requires a period of 20 hours to make the polyimidecompound uniformly conductive in the step, but the period issignificantly shortened compared to the period of at least a half yearrequired under normal temperature and humidity condition (temperature:approximately 22° C.; and humidity: approximately 55% RH) to makepolyaniline sufficiently conductive.

The semiconductive polyimide film shown in FIG. 1 is prepared in the twosteps described above.

Next, an image-forming apparatus equipped with an element made of thesemiconductive polyimide film shown in FIG. 1 will be described.

FIG. 4 is a schematic view illustrating an embodiment of theimage-forming apparatus according to the invention.

The image-forming apparatus 100 shown in FIG. 4 is a full-color tandemimage-forming apparatus that forms toner images in respective colors inrespective toner image-forming units rotating in sync with the movementof an intermediate transfer belt by using four toner image-forming unitscorresponding respectively to four color toners in yellow, magenta,cyan, and black, superimposing these toner images on an intermediatetransfer belt (primary transfer) as an intermediate medium, andtransferring the toner images superimposed on the intermediate transferbelt (secondary transfer) onto paper as a recording medium, and fixingthe image thereon.

The image-forming apparatus 100 shown in FIG. 4 has four tonerimage-forming units 110Y, 110M, 110C, and 110K, four primary transferrolls 120, a semiconductive intermediate transfer belt 130 circulatingcounterclockwise supported by multiple supporting rolls 131, a secondarytransfer unit 140 for secondary transfer, and a fixing unit 150 forfixing the unfixed toner image on paper. Among these constituentelements, the intermediate transfer belt 130 is made of thesemiconductive polyimide film shown in FIG. 1.

The four toner image-forming units 110Y, 110M, 110C, and 110K are placedone by one along the circulating direction of the intermediate transferbelt 130, and each toner image-forming unit 110Y, 110M, 110C, or 110Khas a clockwise-rotating photosensitive drum 111. The surface of eachphotosensitive drum 111 is in contact with the surface of intermediatetransfer belt 130. Each primary transfer rolls 120 is placed at theopposite side of the intermediate transfer belt 130 from thephotosensitive drum 111, and the region between the photosensitive drum111 and the primary transfer roll is a primary transfer region.

Each toner image-forming unit 110Y, 110M, 110C, or 110K also has anelectrostatic charging device 112, a developing device 113, and acleaning device 114. The developing device 113 is placed upstream of theprimary transfer region on the circumference of the photosensitive drum111. The electrostatic charging device 112 is placed further upstream ofthe developing device 113. In addition, the cleaning device 114 isplaced downstream of the primary transfer region on the circumference ofthe photosensitive drum 111.

The surface of the photosensitive drum 111 is charged uniformly by theelectrostatic charging device 112. A laser beam corresponding to imageinformation is irradiated from a light emitting device not shown in thefigure onto the surface of the photosensitive drum 111 charged uniformlyby the electrostatic charging device 112, forming an electrostaticlatent image on the photosensitive drum 111 surface. The developingdevice 113 contains a toner, which is charged to a certain polarity inthe developing device. In the developing device 113, the electrostaticlatent image formed on the surface of the photosensitive drum 111 isdeveloped with the toner charged to a certain polarity and a toner imageis formed on the surface of the photosensitive drum 111. A primarytransfer bias having a polarity opposite to that of toner is applied tothe intermediate transfer belt 130 in the primary transfer region by theprimary transfer roll 120. The toner image formed on the surface ofphotosensitive drum 111 is transferred by the action of the primarytransfer bias from the photosensitive drum surface to the intermediatetransfer belt 130 surface. The toner images formed on respective tonerimage-forming units 110Y, 110M, 110C, and 110K are superimposed on theintermediate transfer belt 130, to give a superimposed toner image.

In the primary transfer region, the toner not transferred to theintermediate transfer belt 130 and remaining on the photosensitive drum111 is removed from the photosensitive drum 111 by the cleaning device114.

The secondary transfer unit 140 has a secondary transfer roll 141 placedin contact with the surface of intermediate transfer belt 130 (tonerimage-carrying face), and a backup roll 142 placed at the rear face ofthe intermediate transfer belt 130, and these two rolls 141 and 142 holdthe intermediate transfer belt 130 inside. The region held inside bythese two rolls 141 and 142 is the secondary transfer region. In thesecondary transfer region, a paper P is fed into the secondary transferregion in the same timing when the toner image, which is superimposed onthe intermediate transfer belt 130, reaches the secondary transferregion. A secondary transfer bias having a polarity identical with thatof toner is applied to the backup roll 142. The toner image superimposedon the intermediate transfer belt 130 is transferred by electrostaticrepulsion from the intermediate transfer belt 130 on to the paper P bythe action of the secondary transfer bias. The paper P discharged fromthe secondary transfer unit 140 is conveyed by the conveyor belt 160 toa fixing device 150. The fixing device 150 has a heating roll 151 havinga heating mechanism inside and a fixing belt 152 pressed onto theheating roll 151. The paper P discharged from the secondary transferregion is supplied into the slit between the heating roll 151 and thefixing belt 152. The toner particles in the toner image on paper P arefused by the heating roll 151 and fixed on the paper P under thepressure by the fixing belt 152, forming a fixed toner image thereon.

A belt cleaner 170 is placed downstream of the secondary transfer unit140 for removal of the toner remaining on the intermediate transfer belt130.

Hereinafter, the intermediate transfer belt 130 made of thesemiconductive polyimide film shown in FIG. 1 will be described indetail.

First, method for producing the intermediate transfer belt shown in FIG.4 will be described with reference to FIG. 5.

FIG. 5 is a flow chart illustrating a method for producing theintermediate transfer belt shown in FIG. 4.

For production of the intermediate transfer belt 130 shown in FIG. 4, afilm-shaped polyimide compound formed in Step S11 shown in FIG. 3 isprepared first, and a film having a length approximately required forproducing an intermediate transfer belt 130 is cut off from thefilm-shaped polyimide compound (Step S21). The film-shaped polyimidecompound for use may have a thickness of 0.050 to 0.500 mm, acceptably0.060 to 0.300 mm, and more acceptably 0.060 to 0.150 mm. If thethickness is less than 0.050 mm, it is more difficult to satisfy themechanical properties needed for the intermediate transfer belt, whileif it is more than 0.500 mm, the polyimide compound may cause a problemthat more cracks are formed on the belt surface by the stress derivedfrom deformation of the areas thereof which become in contact with thesupporting rolls 131 shown in FIG. 4. Further, the polyimide compoundused may contain a filler (filler particles) having a particle diameterof 0.1 μm or more. If the particle diameter of the filler is less than0.1 μm, it is almost impossible to raise the mechanical properties,while if it is 0.1 μm or more, it is possible to raise the mechanicalproperties further.

Subsequently, the polyimide compound cut off in Step S21 is wound andsubjected to a high temperature and humidity exposure step (Step S22).In the similar manner to Step S12 shown in FIG. 3, the polyimidecompound is also kept in an environment of a temperature of 45° C. and ahumidity of 85% RH for 20 hours. In this step, the polyimide compoundabsorbs water and the polyaniline is further oxidized to become moreconductive.

The polyimide compound kept under the high-temperature and humiditycondition is then stored in an environment of normal temperature andhumidity, i.e., a temperature of 22° C. and a humidity of 55% RH for 24hours (Step S23). The surface resistivity and the volume resistivity ofthe intermediate transfer belt 130 shown in FIG. 4 may be adjusted bycontrolling the molecular weight of polyaniline and the amount thereofadded in the step of forming a polyimide compound in Step S11 shown inFIG. 3, but the actual surface resistivity and volume resistivity aredetermined after it is kept in an environment of normal temperature andhumidity, i.e., a temperature of 22° C. and a humidity of 55% RH for 24hours. That is, exposure of the polyimide compound which was kept underthe high-temperature and humidity condition to the standard condition,i.e., to normal temperature and humidity condition for 24 hours, allowsrelease of the excessive water used during oxidation of polyaniline fromthe polyimide compound and thus leads the surface resistivity and volumeresistivity of the polyimide compound into a more stabilized range. Therelease of excessive water from the polyimide compound also rectifiesthe length of the polyimide compound, which is elongated during thetreatment, and stabilizes the dimension (shape) of the polyimidecompound in the standard environment, i.e., in normal temperature andhumidity environment.

After the dimension (shape) is stabilized under normal temperature andhumidity condition, the polyimide compound is then cut off in a cutoffstep (Step S24). In this cutoff step, the polyimide compound is cut intoa belt in a desired shape. In the step, the polyimide compound afterSteps S22 and S23 has no dimensional change in normal temperature andhumidity environment and thus can be cut into a correct shape. In thisembodiment, the polyimide compound is cut into a long flat beltproviding a belt including joining regions having a cutoff dimension ofa width of 362.0 mm and a peripheral length of 2110.0 mm.

FIG. 6 is a view illustrating the joining regions at the ends in thelongitudinal direction of the belt cut off in the cutoff step.

As shown in FIG. 6, the terminals 1301 of the belt 1300 in thelongitudinal direction are stamped in a puzzle-cut pattern havingprotuberances 1302 and dents 1303 (cut pattern having mutually engagedmultiple elements that cannot be separated in the horizontal directiononce engaged) in Step S24 of the FIG. 5. The diameter of theprotuberances 1302 and dents 1303 in the puzzle-cut pattern is about 1mm, and the dimension of protuberances 1302 is smaller than that ofdents 1303.

Both terminals 1301 of the belt thus cut are bound.

FIG. 7 is a view illustrating the way connecting two terminals of thebelt shown in FIG. 6.

As shown in FIG. 7, both terminals of the belt 1301 are engaged in thepuzzle-cut pattern, forming a joining region 1304. Then, the joiningregion 1304 is covered with an adhesive tape 1305 having a thermosettingadhesive coated thereon and a release paper further over there, and thejoining region 1304 covered with the adhesive tape 1305 is thenheat-compressed (Step S25) temporarily, and additionally heat-compressedfinally after removal of the release paper of the adhesive tape 1305(Step S26).

In determining the compression temperature in the preliminarycompression-bonding step S25 and the final compression-bonding step S26of FIG. 5, a small test sample of the belt (semiconductive polyimidefilm 1 in FIG. 1) shown in FIG. 9 is withdrawn and subjected to analysisby differential scanning calorimetry (DSC). Results of the differentialscanning calorimetry (DSC) of the semiconductive polyimide film 1 inFIG. 1 for use as the belt shown in FIG. 9 will be described below.

FIG. 8 is a graph showing the results of the differential scanningcalorimetry of the belt shown in FIG. 9 (semiconductive polyimide film 1in FIG. 1).

A heat-flow-rate differential scanning calorimeter DSC-50 manufacturedby Shimadzu Corporation was used for the differential scanningcalorimetry. The sample was heated from room temperature at a heatingrate of 10° C./min, kept at 30° C. for 20 minutes, and then heatedfurther to 330° C. The analysis was performed in air without use ofnitrogen gas. The temperature and weight-decrease rate data weredetermined at an interval of 1 second.

In the graph shown in FIG. 8, the temperatures are plotted on theabscissa axis, and the DSC values (mW) on the ordinate axis. In thegraph shown in FIG. 8, drawn is a differential thermal curve havinginflection points corresponding to the intersections of the tangentlines at the points where the slope of curves changes from increase todecrease and where the slope of curve changes from decease to increase,or, corresponding to the intersections of the tangent lines at thepoints where the slope of curve changes from decease to increase andwhere the slope of curves from increase to decrease. The differentialthermal curve shown in FIG. 8 has four inflection points in the measuredtemperature range of 150° C. or more, and among the temperatures ofthese four inflection points, the temperature closest to 150° C. isapproximately 175° C. (temperature of the inflection point dp in FIG.8). The belt shown in FIG. 9 becomes almost absolutely dry when heatedto a temperature of 175° C. or more and should be subjected to Step S22in FIG. 5.

For prevention of the decrease, the polyimide compound isheat-compressed at a temperature of 175° C. or less, either in thepreliminary heat compression in Step S25 shown in FIG. 5 or the finalheat compression in Step S26. That is, the compound is heat-compressedat a temperature of 140° C. for 1 minute in the preliminaryheat-compression step S25 and at a temperature of 160° C. for 8 minutesin the final heat-compression in step S26, and in this manner, the beltis subjected to an endless processing without damaging the dimensionalstability of the belt. The thermocompression is carried out under normaltemperature and humidity condition in either case.

FIG. 9 is a perspective view illustrating the intermediate transfer belt130 produced after respective steps shown in FIG. 5. The actual joiningregion 1304 is formed at an angle of 4° with respect to the widthdirection of the intermediate transfer belt 130 as shown in FIG. 9.

The endless belt having both terminals thermally compressed togetherwith a thermosetting adhesive immediately after preparation has thejoining region 1304 and the neighboring area shrunk in the widthdirection and waved on the surface, because the water in the joiningregion 1304 and the neighboring area is evaporated by the heat ofthermocompression. The endless belt shrunk in the joining region 1304and the neighboring area and having a waved surface is placed undernormal temperature and humidity condition for 12 hours (Step S27). Inthis way, the joining region 1304 and the neighboring area reabsorbwater in the environment and the shrunk joining region 1304 and theneighboring area return to the favorable dimensional accuracy.

The endless intermediate transfer belt 130 shown in FIG. 4 is producedin Steps S21 to S27 described above.

The front and rear surfaces of the intermediate transfer belt thusproduced may be mechanically or chemically processed as needed. Aroughened surface may be formed, for example, by sand blast method asthe mechanical means or by chemical etching method as the chemicalmeans. On the contrary, a smooth surface may be formed as needed bymeans of, for example, polishing.

The intermediate transfer belt 130 shown in FIG. 9 has favorabledimensional accuracy without cockles or the like caused by shrinkage inthe joining region (seam region) 1304.

The surface resistivity of the intermediate transfer belt 130 shown inFIG. 9 is 10¹⁰ or more and 10¹⁴ Ω/□ or less and acceptably 10¹¹ Ω/□ ormore and 10¹³ Ω/□ or less. If the surface resistivity is larger than10¹⁴ Ω/□, electrical exfoliation discharges occur more frequently in thepost nip area where the photosensitive drum 111 and the intermediatetransfer belt 130 shown in FIG. 4 are exfoliated, leading to deletion ofimages in the electrically discharged area. In contrast, if the surfaceresistivity is less than 10¹⁰ Ω/□, the electric field strength in thepre-nip portion becomes too high, frequently causing gap discharges inthe pre-nip area and occasionally worsening the graininess in imagequality. Therefore, adjustment of the surface resistivity of theintermediate transfer belt in the range above allows prevention ofdeletion of images caused by the discharge generated when the surfaceresistivity is higher and the deterioration in image quality when thesurface resistivity is lower.

In addition, the volume resistivity of the intermediate transfer belt130 shown in FIG. 9 is 10⁸ Ω·cm or more and 10¹⁴ Ω·cm or less and may be10⁹ Ω·cm or more and 10¹³ Ω·cm or less. If the volume resistivity isless than 10⁸ Ω·cm, the electrostatic force for retaining the electriccharge on the unfixed toner image on the intermediate transfer belt 130transferred from the photosensitive drum 111 becomes smaller,occasionally resulting in scattering of the toner particles aroundimages (blur) and formation of images greater in noise, which are causedby the electrostatic repulsion among toner particles or the fringeelectric field surrounding the image edge. In contrast, if the volumeresistivity is higher than 10¹⁴ Ω·cm, a discharging mechanism will beneeded, as the intermediate transfer body surface is charged by thetransfer electric field during primary transfer due to its highcharge-retaining capacity. Therefore, adjustment of the volumeresistivity in the range above allows prevention of the scattering oftoner particles and provides a solution to the problem of requiring adischarging mechanism.

Hereinafter, an image-forming apparatus having a configuration differentfrom that of the image-forming apparatus shown in FIG. 4, and employingthe intermediate transfer belt shown in FIG. 9, will be described byusing the same numbers for the constituent elements as those used forthe corresponding elements in the image-forming apparatus shown in FIG.4.

FIG. 10 is a schematic view of an image-forming apparatus equipped witha rotary developing device.

The image-forming apparatus 100 shown in FIG. 10 has an intermediatetransfer belt 130 shown in FIG. 9 and a photosensitive drum 111. Theimage-forming apparatus 100 has only one photosensitive drum 111. Theintermediate transfer belt 130 shown in FIG. 9 is placed in contact withthe surface of the photosensitive drum 111, supported by multiplesupporting rolls 131. The image-forming apparatus 100 has a primarytransfer roll 120 at a position facing the photosensitive drum 111 atthe opposite side of the intermediate transfer belt 130, and the regionwhere the photosensitive drum 111 and intermediate transfer belt 130becomes in contact is the primary transfer region.

A rotary developing device 113 is placed upstream of the primarytransfer region on the circumference of the photosensitive drum 111. Therotary developing device 113 contains developing devices 113K, 113Y,113M, and 113C, which contain color toners respectively in black (K),yellow (Y), magenta (M) and cyan (C). In addition, electrostaticcharging, light emitting, and cleaning devices not shown in the figureare placed on the circumference of the photosensitive drum 111.

A secondary transfer unit 140 is placed downstream of the primarytransfer region on the periphery of the intermediate transfer belt 130.The secondary transfer unit 140 shown in FIG. 10 has the sameconfiguration as that of the secondary transfer unit shown in FIG. 4.

When each of the image signals in four colors, yellow, magenta, cyan andblack, is input to the image-forming apparatus 100 shown in FIG. 10,photosensitive drum 111 is charged uniformly by the electrostaticcharging device in this image-forming apparatus 100, and anelectrostatic latent image is formed on the surface of thephotosensitive drum 111, by irradiating a laser beam corresponding tocyan image signal, among the four kinds of image information, from thelight emitting device onto the photosensitive drum 111. Then, theelectrostatic latent image formed on the surface of the photosensitivedrum 111 is developed by the cyan toner-containing developing device113C in the rotary developing device 113 to form a cyan toner image onthe surface of the photosensitive drum 111. Next, in the primarytransfer region, the cyan toner image on the photosensitive drum 111 istransferred onto the intermediate transfer belt 130 (primary transfer).After the cyan toner image is primary transferred onto the intermediatetransfer belt 130, toner particles remaining on the surface of thephotosensitive drum 111 are removed by the cleaning device.

Subsequently, a magenta toner image is formed in a similar manner on thesurface of the photosensitive drum 111, and primarily transferred ontothe intermediate transfer belt 130 in the primary transfer region sothat the magenta toner image superimposes the cyan toner image primarilytransferred on the intermediate transfer belt 130 earlier.

Further, yellow and black toner images are formed sequentially, over thetoner image primarily transferred on the intermediate transfer belt 130earlier in the primary transfer position. In this way, a toner imagetransferred and superimposed in the order of cyan, magenta, yellow, andblack from the belt surface is formed on the intermediate transfer belt130.

The superimposed toner image is then retransferred to a paper P in thesecondary transfer unit 140. An unfixed toner image is thus transferredonto the paper P, and the unfixed toner image-carrying paper P is sentto a fixing device 150. The fixing device 150 shown in FIG. 10 has aheating roll 151 and a pressure roll 153 pressed to the heating roll151. The unfixed toner image-carrying paper P is sent into the slitbetween the mutually facing heating roll 151 and pressure roll 153. Thetoner particles in the toner image on paper P are fused by the heatingroll 151, pressed by the pressure roll 153, and fixed on the paper P,providing a fixed toner image.

The intermediate transfer belt 130 shown in FIG. 9 can also be appliedto an image-forming apparatuses 100 having a rotary developing device113 mentioned in the above.

EXAMPLE

Hereinafter, the invention will be described with reference to EXAMPLES,but it should be understood that the invention is not limited by theseEXAMPLE at all.

[Preparation of Semiconductive Polyimide Films]

<Preparation of Polyaniline Solution (A)>

A mixture of polyaniline powder having a certain molecular weight(10,000 to 50,000) and a dopant (an ester having an acid terminalprepared from an organic acid and a polyhydroxy compound) in DMAc isstirred under a nitrogen atmosphere until the mixture becomes a uniformsolution, which is deaerated to give a polyaniline solution (A).

<Preparation of Polyamic Acid Solution (B)>

To DMAC, 4,4-diaminodiphenylether (ODA),3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromelliticdianhydride (PMDA) are added and the resulting mixture is stirred wellunder a nitrogen atmosphere. ODA, BPDA, and PMDA are blended at a molarratio of 1.00:0.55:0.45, to give a polyamic acid solution (B).

<Preparation of Coating Solution (C)>

The polyaniline solution (A) and filler are added to the polyamic acidsolution (B) in different three compositions, and the mixtures arestirred well and deaerated to give coating solutions (C1) to (C3). Thefiller used is an equivalent to ZELEC ECP-3005-XC (brand name,antimony-doped tin oxide having an average particle diameter of 1.0 to3.0 μm) manufactured by du Pont, U.S.

The composition of the coating solution is adjusted in the followingmanner:

The amount of the polyaniline solution (A) added is first adjusted sothat the surface resistivity and volume resistivity of the polyimidefilm after casting fall in a desired value, and other solid matters areadded so that the weight ratio of polyimide:polyaniline:ZELEC becomes100:28:14 in the coating solution, to give a coating solution (C1).

In a similar manner, coating solution (C2) is prepared at a ratio ofpolyimide:polyaniline:ZELEC of 100:21:14, and coating solution (C3),100:35:14. In either coating solution, the amount of ZELEC contained inthe polyimide film after casting is kept at a level where the resultingfilm does not become conductive.

<Preparation of Polyimide Film (D)>

Long films are formed by extruding the coating solutions (C1) to (C3)thus prepared via a T die onto a stainless steel endless belt and driedat 120° C. for 30 minutes and heated at 300° C. for 15 minutes, to givepolyimide films (D1) to (D3) respectively. The amount of the coatingsolutions (C1) to (C3) is adjusted so that the film thickness of thepolyimide films (D1) to (D3) obtained becomes 0.080 mm.

<Weight-Decrease Rate of Polyimide Films and Conditioning Condition>

Hereinafter, presented are examples related to the humidity conditioningcondition and the weight-decrease rate of the polyimide films (D1) to(D3) prepared.

EXAMPLE 1-1

The polyimide film (D1) obtained by the method above is cut into testsamples in the suitable number and shape, and these samples areconditioned in a constant temperature and humidity bath at 28° C. and85% RH. A sample conditioned at 28° C. and 85% RH is withdrawn one piecea week for four weeks and conditioned in an environment of 22° C. and55% RH additionally for 24 hours, and the surface resistivity and volumeresistivity thereof are determined under the same environment of 22° C.and 55% RH.

R8340A digital ultrahigh-resistance/minute-current ammeter (manufacturedby Advantest Corporation) and UR Probe MCP-HTP12 and Regitape UFLMCP-STO3 having a double-ring electrode structure and a connecting portmodified to fit to the R8340A digitalultrahigh-resistance/minute-current ammeter (both, manufactured by DiaInstruments Co., Ltd.) are used for measurement of the resistivity. Asample is placed on the Regitape UFL MCP-STO3 (having a fluorine resinface) with the measuring face facing upward and in contact with thedouble electrode of the UR Probe MCP-HTP12. The sample is pressed undera uniform load of 19.6±1 N by placing a weight on the UR ProbeMCP-HTP12. Measuring conditions of the R8340A digitalultrahigh-resistance/minute-current ammeter are: charge time: 30 sec;discharge time: 1 sec; and applied voltage: 100V.

When the surface resistivity is designated as ρs and the reading of theR8340A digital ultrahigh-resistance/minute-current ammeter R, and thecorrection coefficient of the surface resistivity of UR Probe MCP-HTP12RCF(S) in determining the surface resistivity, the surface resistivityis expressed by the following formula, wherein RCF(S) is 10.00 accordingto the catalogue “Ohm Meter Series Products” of Mitsubishi ChemicalCorp. (Hereinafter, the surface resistivity is determined similarly.)ρ  s[Ω/▫] = R × RCF(S) = R × 10

In addition, when the volume resistivity is designated as ρv; samplethickness, t (μm); and the reading of the R8340A digitalultrahigh-resistance/minute-current ammeter, R; and the volumeresistivity correction coefficient of the UR Probe MCP-HTP12, RCF(V) indetermining the volume resistivity, the volume resistivity can becalculated according to the following formula, wherein RCF(V) is 2.011according to the catalogue “Ohm Meter Series Products” of MitsubishiChemical Corp. (hereinafter, the volume resistivity is determinedsimilarly).ρ  s[Ω ⋅ cm] = R × RCF(V) × (10,000/t) = R × 2.011 × (10,000/t)

A polyimide film (D1) immediately after preparation (under anenvironment of 28° C./85% RH for 0 hour) is used for reference.

FIG. 11 is a graph showing the measured values of the surfaceresistivity and volume resistivity of the sample of EXAMPLE 1-1.

The abscissa axis of the graph shown in FIG. 11 indicates periods(weeks) conditioned at 28° C./85% RH, while the ordinate axis shows thesurface resistivity (common logarithmic value) (log [Ω/□]) and thevolume resistivity (common logarithmic value) (log [Ω·cm]). In thisgraph, the change in surface resistivity is shown by a dashed line andthe change in volume resistivity by a solid line. As apparent from thisFIG. 11, in an environment of 28° C. and 85% RH, both of the surfaceresistivity and volume resistivity declined rapidly in a week.

Separately, the change in the surface and volume resistivity over timeof the sample conditioned at 28° C. and 85% RH for 1 week is determinedin an environment of 22° C. and 55% RH.

FIG. 12 is a graph showing the change in the surface resistivity andvolume resistivity of a sample conditioned at 28° C. and 85% RH for 1week.

The graph shown in FIG. 12 is drawn in a similar manner to the graphshown in FIG. 11, except that the abscissa axis represents the periods(months) stored in an environment of 22° C./55% RH. The graph shown inFIG. 12 indicates that the surface resistivity and volume resistivityremain almost constant respectively after six months. Therefore, itseems that the surface resistivity and volume resistivity of the sampleare stabilized after conditioning in an environment of 22° C. and 55% RHfor 24 hours. The surface resistivity (common logarithmic value) is 12.0log (Ω/□) and the volume resistivity (common logarithmic value) is 11.3log (Ω·cm) after they reach a sufficiently stabilized region.

Separately, the weight-decrease rate of a sample whose resistivityreached a sufficiently stabilized region is determined.

The weight-decrease rate is determined by thermogravimetric analysis byusing a simultaneous differential thermal and thermogravimetric analyzerDTG-50 manufactured by Shimadzu Corporation. The sample is heated fromroom temperature at a heating rate of 10° C./min, held at 30° C. for 20minutes, and then further heated to 300° C. The analysis is performed inair without use of nitrogen gas. The temperature and the weight-decreaserate data are determined at an interval of 1 second (hereinafter, theweight-decrease rate data are determined similarly).

The weight-decrease rate of the sample whose resistivity reached asufficiently stabilized region is −1.68%.

EXAMPLE 1-2

The weight-decrease rate is determined in a similar manner to EXAMPLE1-1, except that polyimide film (D2) is used replacing the polyimidefilm (D1) of EXAMPLE 1-1.

As a result, the weight-decrease rate of the sample whose resistivityreached a sufficiently stabilized region is −1.4%.

EXAMPLE 1-3

The weight-decrease rate is determined in a similar manner to EXAMPLE1-1, except that polyimide film (D3) is used replacing the polyimidefilm (D1) of EXAMPLE 1-1.

As a result, the weight-decrease rate of the sample whose resistivityreached a sufficiently stabilized region is −1.9%.

EXAMPLE 2

In a similar manner to EXAMPLE 1-1, the polyimide film (D1) obtained iscut into test samples in the suitable number and shape, and thesesamples are conditioned in a constant temperature and humidity bath at45° C. and 85% RH. From the samples conditioned at 45° C. and 85% RH, asample is withdrawn at an interval of 3 hours during the first 24 hoursand at an interval of 24 hours after then till a total of 168 hours (1week) and conditioned in an environment of 22° C. and 55% RHadditionally for 24 hours, and the surface resistivity and volumeresistivity thereof are determined under the same environment of 22° C.and 55% RH.

A polyimide film (D1) immediately after preparation (under theenvironment of 45° C./85% RH for 0 hour) is used for reference.

FIG. 13 is a graph showing the measured surface resistivity and volumeresistivity of the samples of EXAMPLE 2.

The graph shown in FIG. 13 is drawn in a similar manner to the graphshown in FIG. 11, except that the abscissa axis represents the periods(days) during which the samples are conditioned in an environment of 45°C. and 85% RH. As shown in the graph of FIG. 13, the surface resistivityand volume resistivity of the sample decline rapidly in an environmentof 45° C. and 85% RH than in an environment of 28° C. and 85% RH andreach values almost in the stabilized region.

Then, change over time in the surface resistivity and volume resistivityof the sample conditioned at 45° C. and 85% RH for 18 hours over time isdetermined in an environment of 22° C. and 55% RH.

FIG. 14 is a graph showing the measured surface resistivity and volumeresistivity of the sample conditioned at 45° C. and 85% RH for 18 hours.

The graph shown in FIG. 14 is drawn in a similar manner to the graphshown in FIG. 12. As shown in the graph of FIG. 14, the sample had atfirst a surface resistivity (common logarithmic value) of 11.8 log (Ω/□)and a volume resistivity (common logarithmic value) of 11.1 log (Ω·cm),but the surface resistivity (common logarithmic value) rises to 12.0 log(Ω/□) by an increase of approximately 0.2 log (Ω/□) after three months.In addition, the volume resistivity (common logarithmic value) alsorises to 11.3 log (Ωcm) by an increase of approximately 0.2 log (Ωcm).Both the surface resistivity and the volume resistivity remain constantafter then for three months.

Further, thermogravimetric analysis of the sample after six monthsrevealed that it has a weight-decrease rate of −1.65%.

EXAMPLE 2-1

The sample, of which the change over time in the surface resistivity andvolume resistivity is followed in an environment of 22° C. and 55% RHduring a period of six months in EXAMPLE 2 (sample after six months), isconditioned additionally in an environment of 28° C. and 85% RH for 24hours, and the surface resistivity and the volume resistivity aredetermined in an environment of 28° C. and 85% RH.

As described above, the surface resistivity (common logarithmic value)and the volume resistivity (common logarithmic value) of the sample,which reached a stable region after six months, are respectively 12.0log (Ω/□) and 11.3 log (Ω·cm), but decrease to 11.4 log (Ω/□) and 10.7log (Ω·cm) after conditioning in an environment of 28° C. and 85% RH for24 hours. After the sample decreased in resistivity is returned to andconditioned in an environment of 22° C./55% RH for 12 hours, the surfaceresistivity (common logarithmic value) and the volume resistivity(common logarithmic value) are respectively 11.9 log (Ω/□), 11.2 log(Ω·cm), revealing that the surface resistivity and volume resistivityalmost return to the values in stabilized region in an environment of28° C. and 85% RH for 24 hours, although slightly lower than the valuesbefore the conditioning.

EXAMPLE 2-2

The sample (sample after six months), of which the change over time insurface resistivity and volume resistivity are followed for six monthsin an environment of 22° C. and 55% RH in EXAMPLE 2, is then conditionedin an environment of 10° C. and 15% RH for 24 hours and the surfaceresistivity and volume resistivity thereof are determined in anenvironment of 10° C. and 15% RH.

As a result, after room temperature conditioning at 10° C. and 15% RH,the surface resistivity (common logarithmic value) increased to 12.6 log(Ω/□) and the volume resistivity (common logarithmic value) to 11.9 log(Ω·cm). The sample increased in resistivity is further returned to andconditioned in an environment of 22  C./55% RH for 12 hours, and thesurface resistivity (common logarithmic value) and the volumeresistivity (common logarithmic value) measured are respectively 12.0log (Ω/□) and 11.3 log (Ω·cm), indicating that the sample has surfaceresistivity and volume resistivity in the stabilized region similar tothose before the conditioning in an environment of 10° C. and 15% RH for24 hours.

EXAMPLE 3

The film (D1) obtained is cut into test samples in the suitable numberand size in a similar to EXAMPLE 1-1 and a required number of samplesare conditioned in a constant temperature and humidity bath at 60° C.and 85% RH. A sample conditioned at 60° C. and 85% RH is withdrawn at aninterval of 3 hours for 24 hours and conditioned in an environment of22° C. and 55% RH additionally for 24 hours, and the surface resistivityand volume resistivity thereof are determined under the same environmentof 22° C. and 55% RH. A film (D1) immediately after production (under anenvironment of 60° C. and 85% RH for 0 hour) is used for reference. Asshown in FIG. 16, because both the surface resistivity and volumeresistivity decreased rapidly in a shorter term than those in FIG. 13,the sample conditioned at 60° C. and 85% RH for 3 hours is furtherconditioned in an environment of 22° C. and 55% RH for additional 24hours until it reaches the stabilized region. The surface resistivity(common logarithmic value) is 11.9 (Ω/□) and the volume resistivity(common logarithmic value) is 11.2 log (Ωcm) at that time. Theweight-decrease rate as determined by thermogravimetric analysis whenthe sample reaches the stabilized region is −1.66%.

COMPARATIVE EXAMPLE 1

A polyimide film (d) is cut into test samples in the suitable size, andthe surface resistivity and volume resistivity of the samples over timeare determined while the sample is kept and conditioned in anenvironment of 22° C. and 55% RH.

FIG. 15 is a graph showing the measured surface resistivity and volumeresistivity of the samples of COMPARATIVE EXAMPLE 1.

The graph shown in FIG. 15 is drawn in a similar manner to the graphshown in FIG. 11, except that the abscissa axis represents the periods(months) during which the samples are stored in an environment of 22° C.and 55% RH.

The graph shown in FIG. 15 indicates that both the surface resistivityand volume resistivity decrease gradually in an environment of 22° C.and 55% RH. Specifically, the surface resistivity (common logarithmicvalue) of the polyimide film immediately after production (conditioningin an environment of 22° C. and 55% RH for 48 hours or less) is 13.4 log(Ω/□), and the volume resistivity (common logarithmic value), 12.7 log(Ω·cm). The weight-decrease rate of the polyimide film immediately afterproduction as determined by thermogravimetric analysis is −0.01 to−0.05%. The surface resistivity (common logarithmic value) of the samplekept and conditioned in an environment of 22° C. and 55% RH for onemonth is 12.8 log (Ω/□); volume resistivity (common logarithmic value),12.1 log (Ωcm), and the surface resistivity (common logarithmic value)of the sample after five months (after a total of six months), 12.0 log(Ω/□); and the volume resistivity (common logarithmic value), 11.3 log(Ω·cm). The results of EXAMPLES 1 and 2 indicate that taking intoaccount the fact that the surface resistivity (common logarithmic value)of the polyimide film in the stabilized region is 12.0 log (Ω/□); thevolume resistivity (common logarithmic value), 11.3 log (Ω·cm), it takesapproximately six months for both the surface resistivity and volumeresistivity to reach a certain stabilized region in an environment of22° C. and 55% RH.

In addition, the weight-decrease rate of the sample kept and conditionedin an environment of 22° C. and 55% RH for six months as determined bythermogravimetric analysis is −1.66%.

The results above indicate that the surface resistivity and volumeresistivity of a polyimide film after preparation of the polyimide film(after imide-ring closure) can be brought into a stabilized region in ashorter period in a high temperature and high humidity environment(e.g., 28° C. and 85% RH or 45° C. and 85% RH) than under normaltemperature and humidity environment (e.g. 22° C. and 55% RH inCOMPARATIVE EXAMPLE 1). It seems that although the polyimide film (d)immediately after production (conditioned in an environment of 22° C.and 55% RH for 48 hours or less) is not provided with sufficientconductivity, storage thereof in an high temperature and high humidityenvironment accelerates oxidation of polyaniline, providing thepolyaniline with conductivity and bringing the surface resistivity andvolume resistivity sooner into the stabilized region. If a polyimidefilm is kept in a high-temperature environment (e.g. 45° C.) withouthumidity, the film is not oxidized sufficiently and does not regain asurface resistivity or a volume resistivity in the stabilized region,even when it is kept in an environment of 22° C. and 55% RH for 12 hoursor more. As the polyimide is hygroscopic as it is and the polyaniline ismore hygroscopic, both the surface resistivity and volume resistivityare affected by the water present in the polyimide film. Accordingly,the surface resistivity and volume resistivity of a polyimide film inthe stabilized region should be determined under the standardenvironment (e.g. 22° C. and 55% RH).

When a polyimide film (d) having a total area of about 200 m² isconditioned for 18 hours with the entire face exposed to air and not incontact with each other in a constant temperature and humidity bath ofabout 30 m³ in volume, controlled at 45° C. and 85% RH, the oxygenconcentration thereof dropped by about 0.5% after the conditioning.

Further, the results in EXAMPLES 2-1 and 2-2 suggest an environmentaldependency that even a sample having the surface resistivity and volumeresistivity in the stabilized region can have different level of thesurface and volume resistivity when stored under a different environment(e.g. 28° C. and 85% RH or 10° C. and 15% RH) and measured under thesame environment. However, the surface resistivity and volumeresistivity of the polyimide film once brought into the stabilizedregion return to the surface and volume resistivity in the originalstabilized region by conditioning under the standard environment (e.g.environment of 22° C. and 55% RH) for 12 hours or more.

In the following example, the conditions for producing an endlessintermediate transfer belt from the polyimide film (d) prepared will bedescribed.

EXAMPLE 4

Polyimide films have generally certain levels of thermal and humidityexpansion coefficients, and thus when processed into a belt, thedimension thereof varies according to the change in environment.Therefore, it is acceptable to condition the film before it is processedinto a belt. The conditioning may be performed either in the conditionshown in EXAMPLE 1 or 2. As it is found that the use of the humidityconditioning condition used for stabilization of surface resistivity andvolume resistivity, particularly the condition of the “conditioning inan environment of 45° C. and 85% RH for 18 hours and subsequently in anenvironment of 22° C. and 55% RH for 24 hours” (hereinafter, referred toas standard humidity conditioning condition), is effective in providingfavorable effects on dimensional stability without a need for additionalhumidity -conditioning step, the polyimide film (D1) is conditionedunder the standard water-content conditioning condition. Then, thepolyimide film (D1) after conditioning is cut into a piece suitable insize for processing a belt in an environment of 22° C. and 55% RH. Asmall piece of polyimide film (D1) prepared during the cutoff isanalyzed by differential scanning calorimetry, and the transitiontemperature thereof is determined from the inflection point closest toand higher than 150° C. in the differential thermal curve.

For differential scanning calorimetry, an instrument identical with thatused for thermogravimetric analysis, i.e., a simultaneous differentialthermal and thermogravimetric analyzer DTG-50 manufactured by ShimadzuCorporation, is used. The sample is heated from room temperature at aheating rate of 10° C./min, held at 30° C. for 20 minutes, and heatedadditionally to 330° C. The analysis is performed in air without use ofnitrogen gas. The temperature and the weight-decrease rate data aredetermined at an interval of 1 second.

The transition temperature of the polyimide film (D1) is 175.2 to 177.8°C. (measured sample number: 5).

Then, both ends of the belt are cut in a puzzle form as shown in FIG. 6,and the terminal puzzle-cut patterns are engaged with each other asshown in FIG. 7 to form a joining region. The joined region is thencovered with an adhesive tape coated with a thermosetting adhesive onthe rear face (width: 5 mm, TB1650, manufactured by Three Bond Co.,Ltd.), and the adhesive tape-covered joined region is thermallycompressed. The thermocompression is carried out by heating and pressingthe joined region by a seal bar. The variation in temperature of theseal bar in the belt width direction is ±10° C. with respect to the settemperature. The set temperature of the seal bar is 160° C. (practically150 to 170° C.) to make the entire width of seal bar at the transitiontemperature of 175.2 to 177.8° C. The period of the thermocompression is480 seconds and the pressure, 0.3 to 0.4 MPa (effective area of sealbar: 10 mm×380 mm). Five intermediate transfer belts having a width of362.0 mm and an inner peripheral length of 2111.0 mm or more areprepared by thermocompression under the conditions above.

The shrinkage of the joining region (seam region) in each of the fiveintermediate transfer belts immediately after the thermocompression isdetermined.

The shrinkage is determined by the difference in the belt width in theseam region before and after thermocompression. For measurement of thebelt width, used is a glass scale with an extra magnifying glass thatallows direct reading at a 0.1 mm scale (hereinafter, shrinkages aredetermined in a similar manner).

The shrinkage is 1.0 to 1.1 mm with respect to a belt width of 362.0 mm.

The amount of waveness in the joined region after thermocompression isalso determined.

After an intermediate transfer belt is stretched by two rods of 28 mm indiameter under a tension of 50 N, the amount of waveness in the beltwidth direction at a distance of 22 mm from the center of the rod whenone side of the joined region (closer to an rod) is located at adistance of 31 mm from the center of one rod (outer diameter: φ28 mm) isdetermined. The amount of waveness is defined as the peak-to-peakdistance of the waves. As the edges of the belt in the width directionoften curl toward inside, measured values in the area within 20 mm fromthe edges are excluded in calculation of the amount of waveness. Theamount of waveness is determined by using a CCD laser displacementsensor LK030 and an amplification unit LK-2000 manufactured by KEYENCECorporation and by moving the CCD laser displacement sensor LK030positioned at 30±5 mm above the belt surface in parallel with the beltsurface (hereinafter, the amounts of waveness are determined similarly).

The amount of waveness is 0.50 to 0.70 mm.

Then, these five intermediate transfer belts are kept in an environmentof 22° C. and 55% RH for 24 hours, and the resulting shrinkages drop to0.2 to 0.3 mm. The amounts of waveness also drop to 0.30 to 0.50 mm.When the belt is installed in the image-forming apparatus shown in FIG.4 and 20% and 30% magenta half tone images are output respectively, noirregularity in density caused by the waveness is observed in theneighborhood of the seam region of intermediate transfer belt. When theintermediate transfer belt thus prepared is placed in the image-formingapparatus and driven for 135k cycles, no accident, such as exfoliation,breakage, or the like, is observed in the seam region. Further, when aregion including the seam region is cut off from the intermediatetransfer belt and the breaking strength (tensile strength) thereof isdetermined, it is confirmed to be approximately 3.8 KN/m, satisfying therequirement in breaking strength for the seam region at 1.0 KN/m.

COMPARATIVE EXAMPLE 2

A polyimide film is thermally compressed by a seal bar set at atemperature of 200° C., which is higher than the transition temperatureof the polyimide film. Considering the variation in temperature of theseal bar, the film is heated practically at a temperature of 190° C. to210° C. In this experiment too, five intermediate transfer beltsidentical in size with that in EXAMPLE 3 are prepared; the shrinkages ofthe respective belt seam regions are determined immediately afterthermocompression; and the shrinkages are 1.5 to 1.6 mm with respect toa belt width of 362.0 mm. The amount of waveness is as high as 0.85 to1.15 mm.

After these five intermediate transfer belts are kept in an environmentof 22° C. and 55% RH for 24 hours, the shrinkages decrease slightly to0.3 to 0.4 mm. In addition, the amounts of waveness also decreaseslightly to 0.75 mm to 0.95 mm. When these five intermediate transferbelts each are placed in the image-forming apparatus shown in FIG. 4 oneby one and 20% and 30% magenta half tone images are output respectivelythereon, the waveness in the neighborhood of the seam regions causesslight irregularity in density in one of the five belts and deletion ofimages in the other four belts. Further, after intensive studies, theirregularity in density and deletion of images are found to occurfrequently when the amount of waveness is more than 0.7 mm.

COMPARATIVE EXAMPLE 3

The temperature of the seal bar is set at 140° C. Five intermediatetransfer belts are prepared in the similar manner to EXAMPLE 4, exceptthat the temperature is set at less than 150° C., practically 130 to150° C., considering the variation in temperature, and the shrinkages ofthe respective belt seam regions are determined immediately afterthermocompression. The shrinkages are 0.5 to 0.6 mm with respect to abelt width of 362.0 mm. The amounts of waveness are 0.40 to 0.50 mm.When the belts are kept in an environment of 22° C. and 55% RH for 24hours, the shrinkages drop to 0.1 to 0.2 mm. The amounts of wavenessdrop to 0.2 to 0.3 mm. When these five intermediate transfer belts eachare placed in the image-forming apparatus shown in FIG. 4 one by one and20% and 30% magenta half tone images are output respectively thereon, noirregularity in density is observed due to the waveness in theneighborhood of the seam regions of the intermediate transfer belts.However, when the intermediate transfer belt thus prepared is placed inan image-forming apparatus and driven for 135k cycles in the similarmanner to EXAMPLE 4, the adhesiveness between the adhesive and the seamregion is lower due to a lower temperature of the seal bar of 140° C.,resulting in accidents such as deformation by the rolls and exfoliationof part of the adhesive during cleaning of the toner with a cleanerblade.

When a belt-joining region is thermally compressed, water in thepolyimide film is evaporated, occasionally causing local shrinkagedeformation in the polyimide film, depending on the temperature ofthermocompression. The local shrinkage deformation in the polyimide filmis likely to contain a reversible aspect derived from the hygroscopicityobservable in the surface resistivity and volume resistivity, and anirreversible aspect that prohibits the film to recover even when storedin an environment of 22° C. and 55% RH for 24 hours. From the results inEXAMPLE 4, it is found that adjustment of the thermocompressiontemperature at a temperature of 150° C. or more and not more than theinflection temperature closest to 150° C. observed in the DTA curve orless allows reduction in thermal shrinkage deformation of the polyimidefilm and provides favorable seam properties.

The entire disclosure of Japanese Patent Application No.2004-087306filed on Mar. 24, 2004 including specification, claims, drawings andabstract is incorporated herein by reference in its entirety.

1. A polyimide film having a polyimide as the primary constituent and containing at least a polymeric electroconductive material, wherein in a weight-decrease vs. temperature-increase curve when the polyimide film is heated gradually from 30° C., a weight-decrease rate at an intersection between a tangent line which passes a first maximum point in the curve having the maximum value in the slope and a tangent line which passes a minimum point in the curve having the minimum value in the slope after the first point, is in the range of −1.9% or more and −1.4% or less.
 2. The polyimide film according to claim 1, wherein the polymeric electroconductive material contains polyaniline and a dopant that makes the polyaniline conductive.
 3. The polyimide film according to claim 1, wherein the rate of the temperature increase is 10° C./min when the polyimide film is heated gradually from 30° C.
 4. An image-forming apparatus for forming an image on a recording medium by forming a toner image on an image carrier and transferring and fixing the toner image finally onto the recording medium, comprising: an intermediate transfer belt in contact with the image carrier whereon the toner image is formed for receiving the toner image from the image carrier once by application of a bias voltage and transferring the toner image onto the recording medium, wherein the intermediate transfer belt is made of a polyimide film having a polyimide as the primary constituent and containing at least a polymeric electroconductive material, and in a weight-decrease vs. temperature-increase curve when the intermediate transfer belt is heated gradually from 30° C., a weight-decrease rate at an intersection between a tangent line which passes a first maximum point in the curve having the maximum value in the slope and a tangent line which passes a minimum point in the curve having the minimum value in the slope after the first point, is in the range of −1.9% or more and −1.4% or less.
 5. The image-forming apparatus according to claim 4, wherein the polymeric electro conductive material contains polyaniline and a dopant for making the polyaniline conductive.
 6. The image-forming apparatus according to claim 4, wherein the rate of the temperature increase is 10° C./min when the polyimide film is heated gradually from 30° C.
 7. The image-forming apparatus according to claim 4, wherein the intermediate transfer belt has a surface resistivity of 10¹⁰ Ω/□ or more and 10¹⁴ Ω/□ or less and a volume resistivity of 10⁸ Ω·cm or more and 10¹⁴ Ω·cm or less.
 8. The image-forming apparatus according to claim 4, wherein the intermediate transfer belt contains filler particles having a particle diameter of 0.1 μm or more.
 9. A method for producing a polyimide film, comprising the steps of: forming a polyimide compound having a polyimide as the primary constituent and containing at least a polymeric conductive agent into a film shape; and exposing to high temperature and humidity the film-shaped polyimide compound formed in the step of forming the polyimide compound wherein the polyimide compound is kept in an environment of a temperature of 27° C. or more and a humidity of 70% or more.
 10. The method for producing a polyimide film according to claim 9, wherein the polymeric electroconductive material contains polyaniline and a dopant for making the polyaniline conductive.
 11. The method for producing a polyimide film according to claim 9, wherein the rate of the temperature increase is 10° C./min when the polyimide film is heated gradually from 30° C.
 12. A method for producing an endless intermediate transfer belt for use in an image-forming apparatus that forms an image on a recording medium by forming a toner image on an image carrier and transferring and fixing the toner image finally on the recording medium, having an intermediate transfer belt in contact with the image carrier whereon the toner image is formed for receiving the toner image from the image carrier once by application of a bias voltage and transferring the toner image onto the recording medium, comprising the steps of: exposing a film-shaped polyimide compound to high temperature and humidity at a temperature of 27° C. or more and a humidity of 70% RH or more, the film-shaped polyimide compound having a polyimide as the primary constituent, containing at least a polymeric electroconductive material and having a differential thermal characteristic in which a differential thermal curve is created, the differential thermal curve at a temperature of 150° C. or more having a plurality of inflection points corresponding to an intersection between a tangent line passing a point of a change from increase to decrease in the slope of the curve and a tangent line passing a point of a change from decrease to increase in the slope of the curve; cutting into a long belt the film-shaped polyimide compound after the step of exposure to high temperature and humidity; and heat-compressing the resultant belt by connecting both ends of the belt cut off in the step of cutting into a long belt with an adhesive and heating and compressing both connected end regions at a temperature in the range of 150° C. or more to the inflection point closest to 150° C. where the slope of the differential thermal curve becomes smaller.
 13. The method for producing an intermediate transfer belt according to claim 12, wherein the polymeric electroconductive material contains polyaniline and a dopant for making the polyaniline conductive. 